Inkjet printers print dots (pixels) by ejecting very small drops of ink onto a print medium (herein generically referred to as “paper”). They often include a movable carriage that supports one or more print heads each having ink ejecting nozzles. The carriage repeatedly passes over the surface of the paper, which is moved incrementally relative to the carriage between passes, and the nozzles are selectively “fired” to eject drops of ink at appropriate times pursuant to commands of a microcomputer or other print controller, the timing of the application of the ink drops corresponding to the pattern of pixels of the image being printed.
There are also so-called page-high (or page-wide, depending on the page orientation) inkjet printers in which the print head is in the form of a printbar extending the full height (or width) of a page to be printed. In this case the printbar has an array of ink ejecting nozzles along substantially its full length, so that an entire page is printed during a single pass of the printbar relative to the page. Again, a print controller determines which nozzles fire and when as the printbar passes over the page. In some cases the printbar moves across the stationary paper; in others, the printbar is stationary and the paper passes below it. These printers are especially useful for the fast printing of monochrome (e.g. black) text, and are used in, for example, monochrome copiers. Other inkjet printers use a printbar which, although not extending the full height or width of a page, extend a substantial part thereof, so that a complete page is printed only after a small number of passes, say two or three.
In inkjet printers, especially those with a large number of nozzles such as page-wide and other printbar printers, the need to eject a number of drops per nozzle, typically of the order of hundreds of firing cycles, in order to “wake up” the nozzle before starting a print job results in a lot of ink wastage compared to the ink used to actually print. This wastage is worse as print jobs are shorter and more spaced in time, and is especially high in text copying, which has a very low print density.
Prior solutions are limited to determining a minimum value of wake-up firing that is applied to all nozzles in a print head.